BBT help please

My fiancé and me have been trying for over 5 years for our first baby. We’ve tried everything.. iv been given metformin from the doctor as he told me I need to lose weight and that I have PCOS (bet haven’t been tested for it) we’ve been rejected fertility twice.. and last year we had a positive test but sadly it ended with me bleeding and cramping badly a few weeks later.

My question. How do I go about do the temp way? What’s the best thermometer to use? Can I get one that will go to my phone?

Sorry for all the questions. This is all new to me and the only thing we haven’t tried.

Thank you in advance